Lead-acid storage batteries are produced in a variety of forms, with the differences in form being largely the result of compromises made between such factors as cost, weight, volume, capacity and service life. All lead-acid storage batteries have the common features of a positive plate in which the active material is lead dioxide, a negative plate composed of lead, and a water-sulfuric acid electrolyte. A typical lead-acid battery consists of a plurality of cells filled with aqueous sulfuric acid electrolyte. The cells employ negative plates using lead as the electroactive material and positive plates in which lead oxide is the electroactive material. A lead-acid battery has lead positive plates, lead oxide negative plates, and an electrolyte which is a solution of sulfuric acid and distilled water. The plates and the plate separator material between them are arranged in plate packs in plate compartments. During charging, some of the water content of the electrolyte will be hydrolyzed into its component hydrogen and oxygen gases. A general lead acid storage battery is made by inserting assembled plate groups into a container and bonding a cover onto the container. The container has generally been made by forming and working synthetic resin into a boxed shape. The adjoining cells have generally been connected to each other by a connecting conductor installed on top of a partition wall of the container. The negative plate is filled with grey oxide incorporating red lead oxide barium sulfate, and carbon black, while the positive plate is filled with grey oxide incorporating red lead oxide or litharge. Both plates are immersed into an electrolytic solution of diluted sulfuric acid. The red lead oxide plate is the positive pole and grey lead plate is the negative pole. In each cell, the anode is porous metallic lead and the cathode is made of lead dioxide. In general lead acid batteries, an electrode plate is formed by coating lead oxide as a positive active material or lead as a negative active material on a grid made of lead or a lead alloy. The positive and negative electrode plates thus formed are positioned to face each other through a separator formed of glass fibers as a primary component, thereby fabricating a group of electrodes. Sets of an equal number of positive and negative plates in the electrolyte are referred to as cells. Each lead-acid cell produces two volts per cell. The assembly of several cells in a series circuit to produce a higher voltage is called a battery.

Based on the quantity of acid present in a battery, lead-acid batteries are classified as either flooded or starved electrolyte batteries. In flooded batteries, acid is in excess which is also referred to as free acid. In starved electrolyte batteries there is no excess or free acid. Starved batteries are designed such that all the acid in the battery is immobilized in the plates and in either the separators or in gel form. Where the acid is immobilized in the separators, the battery is referred to as an absorbed glass matrix (AGM) battery. Where the acid is immobilized in the gel form, the battery is referred to as a gel type battery. For both battery types, the higher the battery specific energy the lower the number of discharge/charge cycles and hence, the battery cycle life. Flooded lead acid batteries designed for starting, lighting and ignition (SLI) experience a variety of rough handling during manufacture, storage and distribution including an occasional accidental tilting of the battery on its side, a variety of angled inclines once the battery is installed within a vehicle, as well as normal vibrations. During normal operation of a battery, water is electrolyzed into hydrogen and oxygen while temperature excursions produce water vapor, both of which will tend to be lost through the battery venting system. A valve regulated lead-acid (VRLA) battery, also known as sealed lead acid battery, is a starved electrolyte AGM (absorbent glass mat) type battery and has a safety valve which prevents excessive build up of gas pressure inside the battery. VRLA batteries have numerous advantages over their counterpart flooded batteries, such as, for example, low maintenance, ease of handling, and high power density. The very low levels of gas discharge from this type battery permit its use for office equipment. Because the electrolyte is held in a matrix, there should be no electrolyte splash or spillage if the cell case is cracked or otherwise damaged. The cell has the potential to operate for some time with a cracked case. VRLA batteries are particularly suited for remote back-up power applications because they do not require the same type of periodic maintenance required by flooded cells. Valve-regulated lead-acid batteries have achieved significant acceptance in recent years as sources of standby electrical power. These absorbent glass mat valve-regulated lead-acid batteries have become widely used to provide standby power for telecommunications applications, typically for cellular phone towers, other telecommunications equipment and computers.

A valve regulated lead acid battery or sealed lead acid battery comprises separators and plates stacked within a sealed container, in which an electrolyte in a cell is retained in the pores of the separators and both of positive and negative electrode plates so as not to flow. Valve-regulated lead-acid batteries are also called sealed or recombinant batteries. Valve-regulated lead-acid (VRLA) batteries rely upon internal gas recombination to minimize electrolyte loss over the life of the battery, thereby eliminating the need for re-watering. Internal gas recombination is achieved by allowing oxygen generated at the positive electrode to diffuse to the negative electrode, where it recombines to form water and also suppresses the evolution of hydrogen. The diffusion of oxygen is facilitated by providing a matrix that has electrolyte-free pathways. In VRLA batteries oxygen, which is generated during charging at the positive electrode, is reduced at the negative electrode. Thus the battery can be charged and even be overcharged without water consumption and is therefore theoretically maintenance-free. The formation of hydrogen at the negative electrode is suppressed at the positive plate before the negative plate is fully charged. VRLA batteries are classified into absorptive glass mat (AGM) and gel batteries. In batteries with AGM, the absorptive glass mat immobilizes the electrolyte and simultaneously functions as a separator. In gel batteries, the acid is immobilized by means of fumed silica and an additional separator is required to fix the plate distance and to prevent electronic shorts. Compared to AGM batteries, the manufacturing cost of gel batteries is considered to be higher and their specific power is lower due to a higher internal resistance. Sealed lead-acid batteries in which oxygen gas generated at the positive electrode during charging of the battery is absorbed by the negative electrode have previously been known in two general categories. One is a retainer type and the other is a gel type. Both are generally considered to be of the acid-starved variety. The retainer-type battery has many advantages such as maintenance-free operation, no electrolyte leakage and attitude independence. Consequently, this battery has recently been used in increasing volume as a power source for portable equipment, cordless convenience devices and computer backups. Compared to the retainer-type, gel-type lead-acid batteries are inexpensive but their life performance has been lower than the retainer-type and open type common lead-acid batteries that use a sufficient amount of electrolyte.

Sealed lead-acid cells are widely used in commerce today. Sealed lead-acid cells utilize highly absorbent separators, and the necessary electrolyte is absorbed in the separators and plates. Accordingly, such cells may be used in any attitude without electrolyte spillage as would occur with a flooded electrolyte lead-acid battery. Such cells are normally sealed from the atmosphere by a valve designed to regulate the internal pressure within the cell so as to provide what is termed an effective "oxygen recombination cycle". The advantages that are provided by sealed lead-acid cells and batteries in comparison to conventional, flooded lead-acid electrolyte batteries are substantial and varied. Sealed lead-acid technology thus offers substantial benefits by eliminating maintenance, expense, environmental concerns, and safety. The sealed lead-acid stationary batteries used for industrial applications where the power requirements are high and quite demanding are typically comprised of from several to a large number of individual sealed lead-acid cells connected to one another to form a battery with the desired capacity and power requirements. The individual sealed lead-acid cells may be connected in series, in parallel or in suitable combinations of series and parallel to form a battery with the desired capacity and power requirements. The successful operation of a VRLA battery relies on the oxygen recombination reaction which prevents the water loss from a battery by recombining the oxygen and by suppressing hydrogen gas liberation at the negative electrode. The starved acid design of the battery facilitates the oxygen recombination reaction. The recombination reactions are facilitated by the starved acid or electrolyte condition where the electrolyte is immobilized in glass separators disposed between the plates of the battery. The recombination process is further enhanced by sealing the cell with a mechanical valve to keep the oxygen from escaping so it has greater opportunity for recombination.